This invention relates to an ink jet printing system.
U.S. Pat. No. 4,555,712 discloses an ink jet printing system comprising: a droplet generator for generating a stream of ink droplets; a supply tank for supplying ink to the generator; a pressure source for applying pressure to the ink in the supply tank to force it to the droplet generator; a gutter for collecting ink droplets not used in printing; a return tank for receiving the ink collected in the gutter; and a pump for pumping ink from the return tank to the supply tank. The printing system further comprises a measurement system for measuring the flow rate of the steam of ink droplets generated by the generator by monitoring the change of ink level in the supply tank when ink is not being pumped from the return tank to the supply tank.
In ink jet printing it is known to increase the rate of flow of ink to a droplet generator without increasing the flow rate of the stream(s) of ink droplets generated by the generator, by providing a bleed connection from the droplet generator. An example of this is disclosed in U.S. Pat. No. 4,555,709.
Ink jet printing systems are cleaned before, after, and, when required, during use by flushing with ink solvent. Solvent flushes out of the droplet generator of the system via its droplet forming nozzle(s). It is known to supply this solvent to the tank which directly supplies the droplet generator during printing, resulting in an undesirable dilution of the printing ink. Solvent is also flushed out of the droplet generator via its bleed outlet, if it has one. It is known to dump this solvent, which wastes solvent, is environmentally unfriendly, and labour intensive in that the dumping is required to be carried out.
According to a first aspect of the present invention there is provided an ink jet printing system comprising: a droplet generator for generating at least one stream of ink droplets; a supply tank for supplying ink to said generator; supply transfer means for driving the ink from said tank to said generator; and measurement means for measuring the flow rate of the stream(s) of ink droplets generated by said generator by monitoring the change of ink level in said tank, said system including a bleed connection from said generator to said tank in closed circuit with said tank, the presence of said bleed connection thereby increasing the rate of flow of ink from said tank to said generator without affecting the measurement by said measurement means of the flow rate of the stream(s) of ink droplets generated by said generator.
According to a second aspect of the present invention there is provided an ink jet printing system comprising: a droplet generator for generating at least one stream of ink droplets; a supply tank for supplying ink to said generator; supply transfer means for driving the ink from said tank to said generator; a holding tank for receiving liquid issued by said generator during preparation of the system for subsequent printing, said holding tank being connected to said supply tank for the supply of said liquid to the supply tank during said subsequent printing; and means for determining during said subsequent printing the times at which the composition of the ink supplied to said generator is such that it is appropriate to use the liquid contained in said holding tank.
According to a third aspect of the present invention there is provided an ink jet printing system comprising: a tank for holding a liquid for use in the operation of said system; a multi-level or continuous level sensor for measuring the rate of change of liquid level in said tank during said operation; and means for using this measurement to provide an indication that there has been a fault in said system.